Tin-lead has been a worldwide solder staple for many decades due to its advantageous low melting range forming a stable joint with no damage to heat-sensitive parts. Recent legislation by the European Union has required the removal of lead from manufactured products via the Directive entitled: “Restriction of Hazardous Substances (RoHS).”
A preferred alternative solder would be a lead-free tin solder. This poses manufacturing problems due to a phenomenon known as tin whiskers. Tin whiskers are individual crystals of tin that, for example, grow spontaneously from a tin coated surface of a part, as shown in FIG. 1A (see: Slide 4 of “Tin Whiskers: A History of Documented Electrical System Failures” by Dr. Henning Leidecker and Jay Brusse, http://nepp.nasa.gov/whisker/reference/tech_papers/2006-Leidecker-Tin-Whisker-Failures.pdf) and in FIG. 1B (see: http://nepp.nasa.gov/whisker/photos/pom/2004april.htm). These tin whiskers can cause deleterious problems in electronic assemblies, such as electrical shorts and, if broken loose, mechanical damage.
Accordingly, there is a need for a lead-free tin solder that prevents the growth of tin whiskers, thereby eliminating the aforementioned problems.